In magnetic recording systems, a rapid increase in the areal density of magnetic media has led to reduction of the spacing between the head and the medium down to less than 10 nm. Maintaining a constant head-to-medium spacing (HMS) is important throughout the life of the magnetic recording system, since the close proximity of the head to the medium makes the drive susceptible to reliability issues that could lead to temporary modulation of the HMS. In addition, it is important to be able to detect and map defects or asperities on the magnetic medium to prevent collisions between the head and medium during operation. Most conventional recording systems do not provide reliable approaches to monitoring the HMS or detecting asperities on the medium in-situ.
The difference in temperature between the head and the medium results in heat transfer during operation, which may be represented by:
                    q        =                              K            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                                h            +                          cT              p                                                          (                  Equation          ⁢                                          ⁢          1                )            where q is the amount of heat transferred between the head and the medium, h is the separation between the head and the medium, p is the pressure at the sensor, c is a constant that depends on the molecular properties of the air surrounding the head and the medium, T is the ambient temperature, K is the conductivity of air, and ΔT is the difference in temperature between the head and the medium. Thus, because the amount of heat transferred between the head and the medium is proportional to ΔT and inversely proportional to h, the temperature at the medium confronting surface of the head may be measured to continuously monitor changes in the HMS.
Some conventional systems attempt to capitalize on this phenomenon by monitoring temperature changes in the reader of the head. However, in order to improve the detection sensitivity of the reader, the reader element had to be biased at a relatively high voltage to sense the change in resistance in the reader element caused by the temperature change. This can lead to compromised reader life and, because the reader is highly magnetoresistive, can also make it difficult to differentiate between the thermally and magnetically induced components of resistance change in the reader.